JP4363345B2 - Hot-rolled steel sheet for hydroforming, its manufacturing method, and electric-welded steel pipe for hydroforming - Google Patents

Description

  The present invention relates to a hot-rolled steel sheet for hydroforming, a manufacturing method thereof, and an electric-welded steel pipe for hydroforming. For example, the present invention relates to a hot-rolled steel sheet for hydroforming, which is formed by hydroforming of an automobile body, and is particularly preferably used as a material such as a structural member or a suspension member, a manufacturing method thereof, and hydroforming It relates to an electric resistance welded steel pipe.

  As is well known, in recent years, there has been a global demand for reducing the total amount of carbon dioxide emissions, particularly in order to prevent global warming. For example, for automobiles, the reduction of exhaust gas by improving fuel efficiency is being strongly promoted. As one of the measures for improving the fuel efficiency of automobiles, further weight reduction of automobile bodies is required. On the other hand, in order to improve the collision safety, the improvement of the impact absorbing ability by the improvement of the rigidity of the automobile body is also required. In order to satisfy these various requirements and achieve both a light weight and a high rigidity of the automobile body at a high level, it is indispensable to further increase the strength and thickness of the steel sheet for the automobile body.

  Under such circumstances, in recent years, it has been studied to manufacture a structural member and a suspension member of an automobile body by hydroforming a steel plate or steel pipe made of high-strength steel. In short, hydroforming is the process of forming a material using liquid pressure instead of a die. Specifically, a steel plate or a steel pipe is fixed with upper and lower molds, and the material is swollen by liquid pressure. It is the method of making it shape | mold and let it adapt to a type | mold. If it becomes possible to supply structural members and suspension members manufactured by hydroforming, weight and cost can be reduced by reducing the number of parts and welded parts, and the rigidity of the car body can be reduced. Since it can be increased, a great merit can be expected, such as an improvement in collision safety.

  In order to make full use of the merits of such hydroforming, a material suitable for this is required. Specifically, it is important to use a material that does not cause cracks during bulge processing in hydroforming and does not increase the processing cost as much as possible.

  Patent Documents 1 to 3 disclose steel sheets having a ferrite-based structure or a ferrite single-phase structure as a material excellent in hydroformability. For this reason, in Patent Document 1, if there is a hard second phase other than ferrite (for example, pearlite, martensite, or cementite) in the material, the material is soft at a relatively early stage of plastic deformation during hydroforming. This is because cracks starting from the interface between the ferrite and the hard second phase occur, and in Patent Documents 2 and 3, the ductility is not deteriorated. As described above, Patent Documents 1 to 3 disclose that it is desirable to use a ferrite-based structure in order to improve hydroformability.

In addition, Patent Document 4 discloses that hydroform processing is applied to a material that has been provided with precipitation strengthening ability by adding Cu to a low carbon steel material in order to ensure sufficient strength while making the material of hydroform processing mainly composed of ferrite. An invention has been disclosed in which strength is ensured using solid solution strengthening or precipitation strengthening of ferrite by performing a heat treatment after performing the above.
Further, Non-Patent Document 1 discloses a steel pipe having a high hydroforming property with an aging index of 10 MPa or less.

Japanese Patent Laid-Open No. 2001-32034 JP 2000-119812 JP 2002-69584 A JP 2001-303193 A CAMP-ISIJ vol.17 (2004) 505

  According to the inventions described in Patent Documents 1 to 3, simply forming a ferrite-based structure improves hydroformability, but it is difficult to increase the strength of the material. May be difficult.

  Further, in the invention disclosed in Patent Document 4, it is necessary to perform heat treatment in order to strengthen precipitation in structural members and suspension members manufactured by hydroforming. For this reason, in this invention, the manufacturing process of a structural member or a suspension member becomes complicated, and the manufacturing cost increases. As is well known, structural members and suspension members of automobile bodies are also strongly required to be low in manufacturing cost. From this point of view, it is difficult to actually implement the present invention.

  Furthermore, in the invention disclosed in Non-Patent Document 1, since the ferrite single-phase structure is not used, the interface between the soft ferrite and the hard second phase is relatively early in the plastic deformation during hydroforming. Cracks are easily generated from the starting point.

  Thus, according to the conventional invention, if the tensile strength is reduced with the steel sheet or steel pipe as the material as the main structure of ferrite, it is surely possible to prevent the occurrence of breakage during the bulging process in the hydroforming process. Naturally, the strength of the material is reduced, although it becomes possible. For this reason, since it is necessary to increase the strength of structural members and suspension members of an automobile body manufactured by hydroforming by performing heat treatment after hydroforming as disclosed in Patent Document 4, the structure of an automobile body The manufacturing cost of the member and the suspension member increases.

The present inventors investigated the influence of the composition on hydroformability of a large number of steel sheets having different compositions and production conditions and ERW steel pipes made of these steel sheets. As a result, the C content is over 0.01% and 0.1% or less (in this specification, “%” means “mass%” unless otherwise specified), and Ti is over 0.03% and 0.2% or less. Add Nb 0.002
% To 0.05% by weight, hot-rolled steel sheet for hydroforming and hydroforming with strength of 500MPa or more and excellent hydroforming properties that do not easily cause cracks during hydroforming Knowing that there is no need to perform heat treatment after hydroforming, especially if structural parts and underbody members of automobile bodies are manufactured by hydroforming them. The present invention has been completed.

  In the present invention, C: more than 0.01% and 0.1% or less, Si: 0.01% or more and 0.5% or less, Mn: 0.1% or more and 2.0% or less, P: 0.04% or less, S: 0.03% or less, Al: 0.001% or more and 0.1% Ti: more than 0.03% and 0.2% or less, Nb: 0.002% or more and 0.05% or less, N: 0.01% or less, and (a) Formula: −0.15 <4 (C + N) − (Ti + V + Nb / 2 + Mo / 2) <0.2 And having a steel composition consisting of the balance Fe and impurities, the tensile strength is 500 (MPa) or more, and the tensile strength (MPa) × uniaxial tensile elongation (%) is 13500 (MPa ·%) or more. This is a hot-rolled steel sheet for hydroforming.

  In the hot-rolled steel sheet for hydroforming according to the present invention, (i) V: less than 0.3% and / or Mo: less than 0.3%, and / or (ii) Ca: 0.0002% or more and 0.01 % Or less is desirable.

  From another point of view, the present invention uses the hot-rolled steel sheet for hydroforming according to the present invention described above as a raw material, and the limit expansion ratio in hydroforming under a tube end fixing condition is the formula (b): tensile strength of steel pipe It is an electric resistance welded steel pipe characterized by satisfying (MPa) × limit expansion rate (%) ≧ 6000 (MPa ·%). However, the limit pipe expansion ratio in the equation (b) is obtained as {(rupture portion steel pipe circumference-base pipe circumference) / element pipe circumference} × 100.

  In the present invention, the term “hydroformed steel sheet for hydroforming” means an electric resistance steel pipe as a raw material when being formed or processed by the pressure of a liquid. For example, an electric resistance steel pipe used for tube hydroforming is exemplified. Is done.

From another point of view, the present invention performs hot rolling with a steel ingot or steel slab such as a continuous cast slab having the above-described steel composition at 1150 ° C. or higher, and (Ar ₃ points + 100 ° C.) or lower ( Ar ₃ points-50 ° C) Hot-rolled steel sheet for hydroforming, characterized in that after hot rolling is completed, the time until winding is 3 to 60 seconds, and winding is performed at 300 ° C to 700 ° C. It is a manufacturing method.

  According to the present invention, a hot-rolled hot-rolled steel sheet for hydrofoaming and an electric-welded steel pipe for hydrofoaming having a strength of 500 MPa or more and excellent hydrofoaming properties that hardly cause cracking during bulging by hydrofoaming, It can be provided at low cost without performing heat treatment after hydroforming.

  For this reason, the hot-rolled steel sheet for hydroforming and the electric resistance welded steel pipe for hydroforming according to the present invention are both suitable as materials for structural members of various industrial machines, particularly as materials for structural members of automobile bodies. Can be used.

  BEST MODE FOR CARRYING OUT THE INVENTION The best mode for carrying out a hot-rolled steel sheet for hydroforming according to the present invention, a manufacturing method thereof, and an electric resistance welded steel pipe for hydroforming will be described in detail with reference to the accompanying drawings.

First, the fundamental examination result regarding the plastic deformation behavior at the time of hydroforming of a steel plate and a steel pipe, which is the basis of the present invention, will be described.
(a) FIG. 1 is an explanatory view showing the status of a hydroform forming test of a steel sheet. In this hydroform molding test machine 1, a hydraulic bulge test piece 4 is fixed by a die 2 and a holder 3, and a hydraulic bulge test piece is generated by the pressure of the liquid 5 supplied to the inside of the hydraulic bulge test piece 4. 4 is formed with an overhang having a diameter of 100 mm. The limit overhang height of the overhang portion of the hydraulic bulge test piece 4 in this hydroform forming test is generally positively correlated with the uniaxial tensile elongation of the steel plate that is the hydraulic bulge test piece 4.

  (b) FIG. 2 is an explanatory view showing a strip 50φ spherical head protruding test piece 6. FIG. The limit overhang height of the steel plate by this strip 50φ spherical head overhang test piece 6 and the limit tube expansion rate under the tube end fixing condition of the steel pipe {(ruptured part steel pipe circumference-element tube circumference)} × 100 is generally positively correlated.

  (c) Therefore, the hydroformability of the steel sheet can be evaluated by uniaxial tensile elongation, and the hydroformability of the steel pipe can be evaluated by the height of the strip 50φ spherical head protrusion. For this reason, a steel plate with good hydroformability means a steel plate having a large uniaxial tensile elongation and a high strip 50φ spherical head overhang height.

  (d) Swelling deformation in hydroforming under the condition that the pipe end of the steel pipe is fixed is plane strain deformation in which deformation in the longitudinal direction of the steel pipe is constrained, and in plane strain deformation, triaxial stress is applied from the initial stage of deformation. appear. For this reason, if a large amount of hard inclusions such as cementite, martensite and pearlite and coarse inclusions such as TiN-based particles are present, cracks will occur at a relatively early stage starting from the interface between these and soft ferrite. Occurs.

Next, the effects on hydroformability were investigated by producing steel sheets with different chemical components and production conditions and ERW steel pipes made from the steel sheets, and the following findings were obtained.
(e) In steel pipes, ductility and hydroformability deteriorate due to aging of strain introduced during pipe making.

  (f) By adjusting the content of precipitated elements such as Ti and Nb, and C and N to the necessary and sufficient amounts for the formation of carbides, nitrides and carbonitrides, and by optimizing the cooling conditions, It becomes a ferrite single phase (ferrite volume ratio of 95% or more), and further, a large amount of fine Ti and Nb carbides on the order of several nanometers to several tens of nanometers is precipitated to form a steel sheet having high strength.

  (g) In addition, since it is a ferrite single phase, ductility and hydroformability are improved, resulting in a high hydroform hot-rolled steel sheet having both high strength and high formability, and solid solution C and solid solution N are precipitated. Since it is fixed by an object, there is almost no deterioration in ductility and hydroformability due to strain aging after pipe making.

  Based on the above examination results (a) to (g), C: more than 0.01% and 0.1% or less, and steel grades with a very low C content, Ti: more than 0.03% and 0.2% or less, Nb: 0.002% or more and 0.05% or less If the time from finish rolling to winding is 3 to 60 seconds and the winding temperature is 300 to 700 ° C, the strength will be 500 MPa or more, and cracking will occur even during hydroforming. It is possible to provide a hot-rolled steel sheet for hydroforming that has excellent hydroforming properties that are difficult to perform.

Next, the reasons for limitation such as the composition of the hot-rolled steel sheet for hydroforming according to the present embodiment will be described.
C: more than 0.01% and 0.1% or less C is an element necessary for precipitation strengthening by carbides. In order to secure a high strength of 500 MPa or more using precipitation strengthening, C is contained more than 0.01%. However, if the C content exceeds 0.1%, the hydroformability deteriorates due to an increase in the volume fraction of the hard second phase. Therefore, in the present embodiment, the C content is limited to more than 0.01% and not more than 0.1%. From the same viewpoint, the upper limit of the C content is preferably 0.08%, and the lower limit is preferably 0.03%.

Si: 0.01% to 0.5%
Si is an element effective for improving the balance between strength and elongation, and is contained in an amount of 0.01% or more in order to obtain such an effect. However, if the Si content exceeds 0.5%, a Si-based oxide is generated in the weld metal part during welding, and this Si-based oxide becomes a nucleus and easily causes weld metal cracking. Therefore, in the present embodiment, the Si content is limited to 0.01% or more and 0.5% or less. From the same viewpoint, the upper limit of the Si content is desirably 0.3%, and the lower limit is desirably 0.05%.

Mn: 0.1% to 2.0%
When Mn is contained in an amount of 0.1% or more, the strength of the steel is increased. However, if the Mn content exceeds 2.0%, the volume fraction of the second phase increases and the hydroformability deteriorates. Therefore, in the present embodiment, the Mn content is limited to 0.1% or more and 2.0% or less. From the same viewpoint, the upper limit of the Mn content is desirably 1.5%, and the lower limit is desirably 0.3%.

P: 0.04% or less P is an element having an effect of promoting aging and easily segregating. When it is contained in a large amount, the workability is deteriorated. In particular, when the content exceeds 0.04%, segregation becomes remarkable and the workability is greatly reduced. Therefore, in the present embodiment, the P content is limited to 0.04% or less.

S: 0.03% or less S, in order to produce a sulfide to deteriorate the hydroformed property, it is necessary to reduce as much as possible. However, the S content is limited to 0.03% or less in the present embodiment in consideration of the degree of improvement in hydroformability by addition of other components in the present invention and the cost in the steelmaking process.

Al: 0.001% to 0.1%
Al is useful for deoxidizing steel by containing 0.001% or more. However, if the Al content exceeds 0.1%, coarse alumina inclusions increase and hydroformability deteriorates, and Al-based oxides increase in the weld metal during welding. Prone to cracking. Therefore, in the present embodiment, the Al content is limited to 0.001% or more and 0.1% or less. From the same viewpoint, the upper limit of the Al content is desirably 0.07%, and the lower limit is desirably 0.01%.

N: 0.01% or less N causes the formation of coarse TiN-based particles in the steel making process or the casting process to deteriorate the hydroformability. Moreover, when it exists as solid solution N in a steel plate, the formability after pipe making will deteriorate remarkably by strain aging deterioration. Furthermore, when it contains abundantly, the solid solution N will increase, and the strength will decrease due to the decrease in the amount of Ti that contributes to strengthening. Therefore, in the present embodiment, the N content is limited to 0.01% or less. From the same viewpoint, the upper limit of the N content is preferably 0.006%.

Ti: more than 0.03% and less than 0.2%
Ti contributes to improvement and strengthening of aging resistance by reducing solid solution N and solid solution C by precipitation of TiN-based particles and TiC-based particles. When the Ti content is 0.03% or less, a large amount of TiN particles are re-dissolved during heating and the aging resistance deteriorates. On the other hand, if the Ti content exceeds 0.2%, the ductility deteriorates, and at the same time, the TiN-based particles become coarse and the hydroformability also decreases. Therefore, in the present embodiment, the Ti content is limited to more than 0.03% and 0.2% or less. From the same viewpoint, the upper limit of the Ti content is preferably 0.17%, and the lower limit is preferably 0.035%.

Nb: 0.002% to 0.05%
Nb mainly contributes to precipitation strengthening, like Ti. Furthermore, since the precipitation temperature ranges of Ti and Nb are different, by adding Ti and Nb in combination, precipitation during cooling is completed in a short time, and solid solution C and solid solution N are efficiently reduced. Can improve aging resistance. On the other hand, it also has the effect of suppressing softening of the weld heat affected zone during pipe making. These effects cannot be obtained when the Nb content is less than 0.002%. However, if the Nb content exceeds 0.05%, the ductility deteriorates and the cost increases because it is more expensive than Ti. Therefore, in this embodiment, the Nb content is limited to 0.002% or more and 0.05% or less. From the same viewpoint, the upper limit of the Nb content is preferably 0.04%, and the lower limit is preferably 0.01%.

V: less than 0.3%, Mo: less than 0.3% V, Mo are all a precipitation strengthening element like the Ti be any additive element, is effective in improving the strength. However, when the respective contents are 0.3% or more, the ductility deteriorates and the cost is increased because it is more expensive than Ti. Therefore, in the present embodiment, it is desirable that the V content is less than 0.3% and the Mo content is less than 0.3%.

Ca: 0.0002% to 0.01%
Ca is an optional additive element and is added for the purpose of further improving the hydroformability. Ca exists as an oxide in the molten steel and becomes the precipitation nucleus of TiN-based particles. To refine the TiN-based particles, the cracks originating from these TiN-based particles are reduced and the hydroformability is improved. In order to exhibit such an effect, Ca is contained by 0.0002% or more. However, if the Ca content exceeds 0.01%, the oxide in the weld metal part during welding is increased, and it becomes easy to cause a weld crack starting from the oxide. Therefore, in the present embodiment, the Ca content is limited to 0.0002% or more and 0.01% or less. “TiN-based particles” are particles containing Ti and N generated during the steelmaking stage, in molten steel, and in the solidification process of slabs, and are expressed as so-called (Ti, Nb) N containing Nb. Including.

−0.15 <4 (C + N) − (Ti + V + Nb / 2 + Mo / 2) <0.2
In the hot-rolled steel sheet for hydroforming according to the present embodiment, the contents of C, N, Ti, Nb, V, and Mo satisfy −0.15 <4 (C + N) − (Ti + V + Nb / 2 + Mo / 2) <0.2. . Here, each element symbol indicates the content (% by mass) of each element.

  Between the end of finish rolling and winding, ferrite transformation and carbide precipitation become active, and excess C precipitates as cementite, pearlite, and the like. At this time, if the contents of C, N, Ti, Nb, V and Mo satisfy −0.15 <4 (C + N) − (Ti + V + Nb / 2 + Mo / 2) <0.2, the obtained hot rolling for hydroform processing is obtained. The ferrite volume fraction of the steel sheet is 95% or more. If the value of 4 (C + N)-(Ti + V + Nb / 2 + Mo / 2) is less than 0.2%, it becomes a substantially ferrite single phase (ferrite volume fraction of 95% or more), improving ductility and hydroformability, and not causing strain aging deterioration. . On the other hand, when the value of 4 (C + N)-(Ti + V + Nb / 2 + Mo / 2) is -0.15 or less, precipitation elements such as Ti and Nb are solid-dissolved and the ductility deteriorates, and the coarsening of the precipitate is likely to occur. Hydroformability deteriorates.

Therefore, in this embodiment, the value of 4 (C + N) − (Ti + V + Nb / 2 + Mo / 2) is limited to more than −0.15 and less than 0.2. From the same viewpoint, this value is desirably −0.1 or more or 0.15 or less, respectively.
The composition other than the above is the balance Fe and inevitable impurities.

Tensile strength: 500 (MPa) or more, Tensile strength (MPa) x Uniaxial tensile elongation (%): 13500 (MPa ·%) or more The hot-rolled steel sheet for hydroforming according to this embodiment is described in JIS Z 2201. The tensile strength of the No. tensile test piece is 500 (MPa) or more, and the product of the tensile strength (MPa) and the total elongation (%) is 13500 (MPa ·%) or more.

  If the tensile strength of the steel sheet is small, the effect of reducing the weight of the vehicle body and improving the rigidity of the vehicle body is small, and if the elongation is small, the shape that can be formed by hydroforming of the steel sheet is limited. In this embodiment, it is necessary that these two characteristics are balanced at a high level, and the tensile strength is 500 (MPa) or more, and the tensile strength (MPa) × uniaxial tensile elongation (%) is 13500 (MPa).・%) Or more is required.

  In addition, in the ERW pipe made of this steel plate, the product of the steel pipe strength in the No. 11 tensile test piece described in JIS Z 2201 and the limit pipe expansion ratio with the pipe end fixed {(steel pipe strength (MPa ) × Temperature expansion rate (%) when the tube end is fixed} is 6000 (MPa ·%) or more, that is, these two characteristics must be balanced in tube hydroforming. For example, a steel pipe having a tensile strength of 6000 MPa, for example, must have a critical expansion ratio of 10% or more.

Next, the manufacturing method of the hot-rolled steel sheet for hydroforming according to the present embodiment will be described.
First, a steel ingot or steel slab such as a continuously cast slab having the above-described steel composition is set to 1150 ° C. or higher. If the temperature of the steel ingot or steel slab subjected to hot rolling is less than 1150 ° C, the coarse TiC in the steel ingot or steel slab does not sufficiently dissolve but remains coarse and the hydroformability deteriorates Not only that, it is sometimes difficult to ensure a predetermined strength due to the reduction of fine precipitates that precipitate during cooling and contribute to strengthening. Therefore, in the present embodiment, the temperature of the steel ingot or steel slab is limited to 1150 ° C. or higher. From the same viewpoint, it is preferably 1200 ° C or higher, more preferably 1230 ° C or higher. However, in order to suppress re-dissolution of TiN-based particles, the temperature of the steel ingot or steel slab subjected to hot rolling is preferably 1400 ° C. or lower.

  In the present invention, the temperature of the steel ingot or steel slab subjected to hot rolling only needs to be 1150 ° C. or higher, so only the embodiment in which the steel ingot or steel slab lowered to a temperature below 1150 ° C. is heated to 1150 ° C. or higher. In addition, a mode is also included in which the steel ingot after continuous casting or the steel slab after partial rolling is subjected to hot rolling without being lowered to a temperature of less than 1150 ° C.

In this way, hot rolling is performed after the steel ingot or steel slab is brought to a predetermined temperature. Then, the hot rolling is finished at a finishing temperature of (Ar ₃ points + 100) ° C. or less (Ar ₃ points−50) ° C. or more. When the finishing temperature exceeds (Ar ₃ points +100) ° C., the austenite grain size before transformation is large, so that the hardenability of the steel is high, the volume fraction of the second phase is increased, and the hydroformability is deteriorated.

When rolling is performed in a two-phase region of ferrite and austenite with less than ₃ Ar, the ferrite grains are distorted and unevenly processed ferrite tends to remain. However, the finish rolling temperature is equal to be (Ar ₃ -50) ° C. A _three or less Ar, the area ratio of ferrite is small and because generation is less uneven deformed ferrite, degrades the workability There is nothing.

Therefore, in the present invention, the finish rolling temperature during hot rolling is limited to (Ar ₃ points + 100) ° C. or less (Ar ₃ points−50) ° C. or more.
After finishing the finish rolling in this way, it is wound on the coil. The time required from the end of finish rolling to the start of winding is 3 to 60 seconds, and the winding is performed at 300 to 700 ° C. take.

  If the time required from the end of finish rolling to the start of winding is less than 3 seconds, sufficient formation of ferrite will not be obtained and the hydroform of the steel sheet will increase due to an increase in the hard second phase that precipitates at low temperatures. As a result, the aging of the steel pipe is deteriorated due to an increase in solute C and solute N. On the other hand, if the time required from the end of finish rolling to the start of winding exceeds 60 seconds, carbides such as Ti and Nb that precipitate during cooling coarsen and the strength and hydroformability deteriorate. Therefore, in the present embodiment, the time required from the time when finish rolling is finished to the time when winding is started is limited to 3 seconds or more and 60 seconds or less. From the same viewpoint, this time is desirably 10 seconds or more, and desirably 40 seconds or less.

  If the coiling temperature is less than 300 ° C., the ductility and hydroformability deteriorate due to the formation of a hard second phase. On the other hand, when the coiling temperature is higher than 700 ° C., the hydroformability deteriorates due to the coarsening of the carbide, and the strength also decreases. Therefore, in this embodiment, the coiling temperature is limited to 300 ° C. or more and 700 ° C. or less. From the same viewpoint, the lower limit of the winding temperature is preferably 350 ° C, more preferably 400 ° C, and the upper limit of the winding temperature is preferably 650 ° C, more preferably 600 ° C. .

  In this way, according to the present embodiment, hot rolling for hydrofoaming having a strength of 500 MPa or more and excellent hydrofoaming properties that do not cause cracking during bulging by hydrofoaming. It is possible to provide a steel plate and an electric resistance welded steel pipe for hydroforming at low cost without performing heat treatment after hydroforming.

Furthermore, the present invention will be described more specifically with reference to examples.
Sixteen steel types A to P having the compositions shown in Table 1 were melted to form slabs. This slab has a sheet thickness of 2.0 mm under the manufacturing conditions shown in Table 2 (slab heating temperature, finishing temperature, time required from the completion of finish rolling to the start of winding (* 2), winding temperature). A hot-rolled steel sheet was obtained.

This hot-rolled steel sheet was formed into a cylindrical shape so that the rolling direction was the pipe axis direction, and the seam portion was subjected to electric resistance welding to obtain an ERW steel pipe having a diameter of 60.5 mm and a wall thickness of 2.0 mm.
The mechanical properties of the hot-rolled steel sheet are determined by cutting out the No. 5 tensile test piece specified in JIS Z 2201 from the direction perpendicular to the rolling direction and conducting a tensile test at room temperature to obtain the tensile strength and total elongation. It was measured.

  In addition, the strip 50φ spherical head overhang test piece shown in FIG. 2 is cut out so that the long side h is perpendicular to the rolling direction, and the configuration of the main part is extracted and shown in FIG. 3 (a). Using a ball head overhang test machine 7, a strip 50φ ball head overhang test was conducted. In FIG. 3 (a), reference numeral 8 indicates an upper mold, reference numeral 9 indicates a lower mold, reference numeral 10 indicates a ball-head punch having a diameter of 50 mm, and reference numeral 11 indicates a strip 50φ ball-head protruding test piece. .

  Then, as shown in FIG. 3 (b), the overhang height when the crack 12 occurred in the strip 50φ spherical head overhang test piece 11 was measured. In addition, the hydroform forming of the steel sheet was performed by injecting water from below using the hydroform forming test machine 1 shown in FIG. 1 to stretch the steel sheet, and measuring the overhang height at this time.

  On the other hand, regarding the mechanical properties of the electric resistance welded steel pipe, No. 11 tensile test piece defined in JIS Z 2201 was cut out and subjected to a tensile test at room temperature to measure tensile strength and total elongation. Hydroform molding is performed in the steel pipe 17 with the pipe end of the steel pipe 17 fixed using the upper mold 14 and the lower mold 15 of the hydroform molding test machine 13 for the steel pipe 17 shown in FIG. 4 (a). The steel pipe 17 was swollen into the space 16 formed by the upper and lower molds 14 and 15 by applying internal pressure with water. Then, as shown in FIG. 4 (b), the process was performed until the burst portion 18 was formed in the steel pipe 17, and the circumference of the expanded portion 19 including the burst portion 18 was measured. And the limit pipe expansion rate was calculated | required by the limit pipe expansion rate = (rupture part steel pipe peripheral length-elementary pipe perimeter) / element pipe perimeter x 100 (%).

  Table 2 shows the characteristics of each of the hot-rolled steel sheet and comparative steel sheet manufactured according to the present invention (TS, TS · El), hydraulic overhang height, strip 50φ ball head overhang height), and ERW steel pipe. The characteristics (TS, TS / marginal expansion rate) are shown together.

  As shown in Tables 1 and 2, the steel sheets and ERW steel pipes (test numbers 1 to 12) manufactured to satisfy the conditions specified in the present invention have excellent elongation and limit expansion for each tensile strength. The rate is shown and it turns out that it is suitable as a hot-rolled steel sheet or an electric-welded steel pipe for hydroforming.

  On the other hand, the test numbers 13 to 15 in which the C content, Mn content and Ti content are excessive are low in the elongation or the limit tube expansion ratio in comparison with the present invention example having the same strength level in both the steel plate and the ERW steel pipe. It turns out that the nature is inferior.

  Moreover, in the trial number 16 where Ti addition amount is too small, it can be seen that although the properties of the steel sheet are comparatively excellent, the ductility and the limit pipe expansion rate of the ERW steel pipe after pipe forming are deteriorated.

It is explanatory drawing which shows the condition of the hydroform shaping | molding test of a steel plate. It is explanatory drawing which shows a strip 50phi spherical head protruding test piece. Fig. 3 (a) is an explanatory diagram showing the configuration of the main part of a strip 50φ ball head overhang test machine. Fig. 3 (b) shows a strip 50φ ball head overhang test piece with cracks. It is explanatory drawing shown. FIG. 4 (a) is an explanatory view showing a hydroforming tester for steel pipes, and FIG. 4 (b) is an explanatory view showing a steel pipe having a burst portion.

Explanation of symbols

1 Hydroform molding tester
2 dice
3 Holder
4 Hydraulic bulge specimen
5 liquid
6 Strip 50φ head test piece
7 Strip 50φ ball head tester
8 Upper mold
9 Lower mold
10 ball head punch
11 Strip 50φ head test piece
12 crack
13 Hydroform molding tester
14 Upper mold
15 Lower mold
16 space
17 Steel pipe
18 Burst part
19 Expansion section

Claims (5)

In mass%, C: more than 0.01%, 0.1% or less, Si: 0.01% or more, 0.5% or less, Mn: 0.1% or more, 2.0% or less, P: 0.04% or less, S: 0.03% or less, Al: 0.001% or more, 0.1% Ti: more than 0.03% and 0.2% or less, Nb: 0.002% or more and 0.05% or less, N: 0.01% or less, further satisfies the following formula (a), has a steel composition consisting of the balance Fe and impurities, and has a tensile strength. Is a hot-rolled steel sheet for hydroforming, characterized by a tensile strength (MPa) x uniaxial tensile elongation (%) of 13500 (MPa ·%) or more.
−0.15 <4 (C + N) − (Ti + V + Nb / 2 + Mo / 2) <0.2 (a)   The hot-rolled steel sheet for hydroforming according to claim 1, further comprising, by mass%, V: less than 0.3% and / or Mo: less than 0.3%.   The hot-rolled steel sheet for hydroforming according to claim 1 or 2, further comprising, by mass%, Ca: 0.0002% to 0.01%. Using the hot-rolled steel sheet for hydroforming as set forth in any one of claims 1 to 3 as a raw material, a limit tube expansion rate in hydroforming under a pipe end fixing condition satisfies the following formula (b): ERW steel pipe for hydroforming.
Tensile strength of steel pipe (MPa) x critical expansion ratio (%) ≥ 6000 (MPa ·%) (b)
However, the limit pipe expansion ratio = {(Ruptured part steel pipe circumference-elementary pipe circumference) / element pipe circumference} x 100 A steel ingot or steel slab having the steel composition described in any one of claims 1 to 3 is hot-rolled at 1150 ° C or higher and (Ar ₃ points + 100 ° C) or lower (Ar ₃ points) −50 ° C.) A hot-rolled steel sheet for hydroforming, characterized in that after the hot rolling is completed, the time until winding is 3 seconds or more and 60 seconds or less and winding is performed at 300 ° C. or more and 700 ° C. or less. Production method.

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